1. Field of the Invention
The present invention relates to monitoring devices for air bags or air cushioning apparatus for the protection of vehicle passengers. More particularly, the present invention relates to a differential pressure monitoring device for sensing a predetermined range of operable conditions and a hybrid inflator for a vehicle air bag system.
2. Description of the Prior Art
It was estimated in October 1992 that 7 million air bags had been installed in automotive vehicles in the United States alone. The Surface Transportation Efficiency Act specifies that driver and passenger side air bags must be fitted to all new automobiles by the 1998 model year and to all light trucks by the 1999 model year. While air bags are estimated to reduce fatal accidents by less than 5% when used in conjunction with 3 point safety belts, they are known to reduce moderate-to-severe injuries by 20-29%, thus, justifying their requirement in vehicles.
Presently there are two basic types of air bag inflating systems. First, there is a self-contained pyrotechnic-type inflator which comprises a sensor, a pyrotechnic igniter/inflator and a folded air bag in a pocket or carrier. The mode of operation of such devices is to sense a predetermined structural deformation which causes an impact force within one and one-half milliseconds of the time of the impact force. Immediately following the sensing of deformation, and before 30-35 milliseconds have passed, when the passenger/occupants would begin moving forward inside of the vehicle, the air bag should become sufficiently inflated to arrest the forward motion of the passenger and retain them in the portion of the vehicle least subject to deformation. The disadvantage of the pyrotechnic-type inflator is its high cost and extremely high heat generated inside of the air bag by the pyrotechnic igniter which may cause burn injuries to the exposed parts of the body touched by the hot air bag.
A second type of air bag employs a lower cost hybrid type inflater which comprises a sensor, an igniter/gas generator, a large volume pressure tank loaded with a predetermined inert gas under very high pressure and a folded air bag. The gas in the pressurized tank is heated and expanded from room temperature by the igniter/gas generator before being used to inflate the air bag which is located remote from the pressure tank. The large volume of inert gas when heated is sufficient to completely inflate the air bag as fast as a pyrotechnic-type but at much lower temperatures during the expansion of the gas in the gas tank. The disadvantage of the hybrid-type inflater has been that a large pressure tank is required and a device for monitoring the inert gas pressure in the tank is required to assure that a minimum volume of inert gas is available for heating and inflating the air bag. The advantage to the hybrid-type inflater is its lower cost and the fact that it generates less heat in the air bag.
U.S. Pat. No. 4,049,935 shows and describes a complex pressure monitoring switch for use with a tank of a pressure chamber of the type employed with a hybrid-type inflater. The snap action switch of this prior art device comprises a diaphragm which is captured between two apertured reinforcement members which limit the excursion of the diaphragm when it snaps from one position to the other. The diaphragm forms one end closure of a cylindrical vessel which is filled to a predetermined fixed pressure level during manufacture.
It would be desirable to provide an improved monitoring device for a hybrid-type air bag inflater for filling an air bag with a large amount of moderately heated gas that is more sensitive to pressure changes in the pressure chamber and may be set to a desired threshold level rather than to a fixed pressure level during manufacture.